Nuclear reprocessing technology has been developed to chemically separate and recover plutonium, uranium, and other actinides from irradiated nuclear fuel. As shown in FIG. 1, pyroprocessing is a high-temperature method that involves introducing spent nuclear fuel into a large scale electrochemical cell, where uranium is first electrochemically dissolved into a molten salt electrolyte and then reductively plated at the cell cathode where it can be collected and ultimately recovered as pure uranium. See H. Lee et al., Sci. and Tech. Nuc. Install. 1 (2013). Left behind in the molten salt electrolyte (e.g., LiCl—KCl), however, are residual fission products, transuranics, and other contaminants in the molten salt electrolyte that can degrade the electrochemical efficiency of the system and pose significant challenges to ultimate waste packaging and disposal. Developing a method to efficiently remove the contaminants would facilitate molten salt recycling and reduce the volume of hazardous waste to be packaged for disposal.
Currently there are several methods under development to recapture and purify the molten salts including zeolite-based ion exchange and melt crystallization. See H. Lee et al., Sci. and Tech. Nuc. Install., 1 (2013); D. Lexa and I. Johnson, Mettalurg. and Mater. Trans. B 32B, 429, (2001); M. Shaltry et al., Micropor. Mesopor. Mater. 152, 185 (2012); and A. Williams et al., Chem. Eng. Sci. 89, 258 (2013). In the ion-exchange process, contaminated molten salt is run through a column containing an alumino-silicate zeolite (e.g., Zeolite-4A), which extracts contaminants such as Sr2+  or Cs+ from the salt. These zeolites can then be sintered into waste forms for disposal. In contrast, melt crystallization purifies the salts by taking advantage of the increased solubility of contaminants in lower temperature eutectic melts. Contaminated salt is melted, and then slowly cooled from the top down, concentrating the contaminants in the liquid melt phase. Once the salt has completely resolidified, the impurities are concentrated in the lowest portion of the solid, which can be removed and prepared for disposal.
The method of the present invention can facilitate the rapid removal of contaminants, and could potentially be integrated into the existing electrochemical system currently used for pyroprocessing of nuclear waste.